Percona MySQL 5.6 WHERE 条件中 OR 的索引测试

在测试表的两列上分别创建索引。
mysql> select * from test;
+------+-------+
| id   | name  |
+------+-------+
|   10 | neo   |
|   20 | John  |
|   30 | Lucy  |
|   40 | Larry |
|   50 | Lilly |
+------+-------+
5 rows in set (0.00 sec)
mysql> show keys from test;
Empty set (0.01 sec)

mysql> create index idx_test_id on test(id);
Query OK, 0 rows affected (0.34 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> create index idx_test_name on test(name);
Query OK, 0 rows affected (0.72 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show keys from test;
+-------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name      | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test  |          1 | idx_test_id   |            1 | id          | A         |           5 |     NULL | NULL   | YES  | BTREE      |         |               |
| test  |          1 | idx_test_name |            1 | name        | A         |           5 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.02 sec)

在OR条件中分别涵盖这两列，会使用到这两个索引。

mysql> explain select * from test where id=10;
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key         | key_len | ref   | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------+
|  1 | SIMPLE      | test  | ref  | idx_test_id   | idx_test_id | 5       | const |    1 | NULL  |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain select * from test where name='Lucy';
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key           | key_len | ref   | rows | Extra                 |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | test  | ref  | idx_test_name | idx_test_name | 18      | const |    1 | Using index condition |
+----+-------------+-------+------+---------------+---------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)

mysql> explain select * from test where id=10 or name='Lucy';
+----+-------------+-------+------+---------------------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys             | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | test  | ALL  | idx_test_id,idx_test_name | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+-------+------+---------------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from test where id=20 or name='Lucy';
+----+-------------+-------+------+---------------------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys             | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | test  | ALL  | idx_test_id,idx_test_name | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+-------+------+---------------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from test where id=50 or name='neo';
+----+-------------+-------+------+---------------------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys             | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | test  | ALL  | idx_test_id,idx_test_name | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+-------+------+---------------------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

删除这两个索引，重建一个索引，在OR条件中会使用到这一个索引。

mysql> drop index idx_test_id;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
mysql> drop index idx_test_id on test;
Query OK, 0 rows affected (0.33 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> drop index idx_test_name on test;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> create index idx_test_id on test(id);
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select * from test where id=50 or name='neo';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | test  | ALL  | idx_test_id   | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from test where name='neo';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | test  | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from test where name='Lucy';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | test  | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

删除表中的索引，创建一个包含这两列的联合索引，在OR条件中会使用到这个索引。发现在MySQL里面联合索引的应用规则和Oracle中不同，单独查询条件中只含一列的WHERE条件，如创建联合索引的字段顺位为A、B，在B上的查询也会使用联合索引。

mysql> drop index idx_test_name on test;
ERROR 1091 (42000): Can't DROP 'idx_test_name'; check that column/key exists
mysql> drop index idx_test_id on test;
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> create index idx_test_id_name on test(id,name);
Query OK, 0 rows affected (0.21 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show keys from test;
+-------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name         | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test  |          1 | idx_test_id_name |            1 | id          | A         |           5 |     NULL | NULL   | YES  | BTREE      |         |               |
| test  |          1 | idx_test_id_name |            2 | name        | A         |           5 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

mysql> explain select * from test where name='Lucy';
+----+-------------+-------+-------+---------------+------------------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key              | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+------------------+---------+------+------+--------------------------+
|  1 | SIMPLE      | test  | index | NULL          | idx_test_id_name | 23      | NULL |    5 | Using where; Using index |
+----+-------------+-------+-------+---------------+------------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select * from test where name='neo';
+----+-------------+-------+-------+---------------+------------------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key              | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+------------------+---------+------+------+--------------------------+
|  1 | SIMPLE      | test  | index | NULL          | idx_test_id_name | 23      | NULL |    5 | Using where; Using index |
+----+-------------+-------+-------+---------------+------------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select * from test where id=10;
+----+-------------+-------+------+------------------+------------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys    | key              | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+------------------+------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | test  | ref  | idx_test_id_name | idx_test_id_name | 5       | const |    1 | Using index |
+----+-------------+-------+------+------------------+------------------+---------+-------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from test where id=10 or name='Lucy';
+----+-------------+-------+-------+------------------+------------------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys    | key              | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+------------------+------------------+---------+------+------+--------------------------+
|  1 | SIMPLE      | test  | index | idx_test_id_name | idx_test_id_name | 23      | NULL |    5 | Using where; Using index |
+----+-------------+-------+-------+------------------+------------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

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